A desirable solar cell geometry referred to as an interdigitated back contact (IBC) cell comprises a semiconductor wafer and alternating lines (interdigitated stripes) coinciding with regions with p-type and n-type doping. This cell geometry has the advantage of eliminating shading losses altogether by putting both contacts on the rear side of the wafer that is not illuminated. Further, contacts are easier to interconnect with both contacts on the rear surface.
Another desirable solar cell architecture involves the use of silicon heterojunction or tunnel junction contacts. A well-known example of such architectures is the HIT (heterojunction with intrinsic thin layer) cell structure. In the conventional front emitter form of this structure, a silicon wafer is contacted on both sides by a thin intrinsic hydrogenated amorphous silicon (a-Si:H) layer, which serves as a surface passivating layer as well as a charge carrier transport layer. On the front of the cell, a semiconductor layer doped to the opposite doping polarity of the base substrate is applied, forming a heterojunction emitter. On the rear of the cell, a semiconductor layer doped to the same doping polarity as the base substrate is applied, forming a base contact. These layers can then be contacted with transparent or metallic conducting layers to extract current from the solar cell. In the tunnel junction cell, the intrinsic a-Si:H layer is replaced with a thin high bandgap material. In the case of the heterojunction cell, charge carrier transport occurs via a band conduction mechanism in the intrinsic a-Si:H layer, while in the case of the tunnel junction cell, charge carrier transport occurs via quantum mechanical tunneling. Despite this difference, these cells operate via similar mechanisms and importantly can be manufactured in low temperature processes because they do not require dopant diffusion.
The conventional heterojunction or tunnel junction solar cells cannot achieve outstanding efficiencies because they still require front side contacts. The presence of a contact on the front side firstly reduces efficiency due to blocking or shading of the incoming light by the necessary metal grids which extract the generated current. Additionally, the presence of a front electrical contact requires that the front of the cell be simultaneously optimized for electrical, light absorption, and passivation properties, often producing a compromise which affects cell performance.
Presently, silicon solar cells with the highest efficiency are those based on combining an interdigitated all back contact structure with silicon heterojunction contacts. Panasonic recently reported obtaining a record conversion efficiency of 25.6% with such a device structure (Masuko et al., 40th IEEE Photovoltaic Specialists Conference, Jun. 8-13, 2014, Denver, Colo.). At the same conference, Sharp reported obtaining an efficiency of 25.1% with a similar device structure (Nakamura et al., 40th IEEE Photovoltaic Specialists Conference, Jun. 8-13, 2014, Denver, Colo.), and SunPower obtained an efficiency of 25.0% with an interdigitated back contact (IBC) silicon solar cell made using conventional diffusion processes (Smith et al., 40th IEEE Photovoltaic Specialists Conference, Jun. 8-13, 2014, Denver, Colo.). While the processing of these high efficiency IBC solar cells were not discussed in any detail, the manufacturing costs are likely to be relatively high since the known processing techniques that could be applied in each case appears to be somewhat complicated with various masking and vacuum processing steps required.
While it is clear that back contact heterojunction emitter solar cells can produce the highest efficiencies, there is a need for improved methods for producing these cells in a manner that eliminates the expense associated with multiple process and alignment steps. Furthermore, there is a need to produce heterojunction or tunnel junction emitter back contact cells with low contact resistance.